To provide and sustain efficient, economical control of a turbocompressor, it is necessary to know both the location and the shape of its surge limit line, as plotted on a performance map. The exactness of this surgelimit information determines how safely a compressor can operate, and it also helps to determine the size of the compressor's operational envelope. Therefore, the more accurately the surge-limit characteristics are estimated, the larger is the region (operational envelope) in which a compressor can function with a closed antisurge valve. However, during normal operation, compressor-system performance characteristics often change significantly due to inherent influences, such as mechanical degradation of its flow-through parts and defects in its seal system that, in turn, cause the location and shape of the surge limit line, as well as the performance limits, to vary.
Establishing the largest possible operational envelope (with the antisurge valve closed) requires compressor testing to identify the actual characteristics of a surge limit, and then using this information to configuring an antisurge controller. A sufficient number of test points must be found to accurately describe the complete interface between stable and unstable operation. Consequently, if a compressor operates within the broad ranges of rotational speed or guide vane position or both of them, the unit must be repeatedly tested using different values of these process variables; as a result, each test leads the compressor into surge which can be detrimental because of strong dynamic loading.